1. Field of the Invention
The present invention relates to a write circuit of a head of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces with separate write and read elements, respectively. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
The disks are rotated by a spindle motor of the drive. Rotation of the disks creates an air flow within the disk drive. Each head has an air bearing surface that cooperates with the air flow to create an air bearing between the head and the adjacent disk surface. The air bearing eliminates or minimizes the mechanical wear between the head and the disk. The height of the air bearing is commonly referred to as the flying height of the head.
The magnetic field detected by the head is inversely proportional to the flying height of the head. Likewise, the strength of the magnetic field written onto the disk is inversely proportional to the fly height. A larger fly height will produce a weaker magnetic field on the disk.
There have been developed heads that include a heater element. Current is provided to the heater element to generate heat and thermally expand the head to move the read and write elements closer to the disk. These types of heads are sometimes referred to as fly on demand (“FOD”) heads. The flying height of FOD heads can be varied by changing the amount of power provided to the heater element.
The write elements of the heads are typically connected to a write driver circuit by a suspension interconnect that contains a number of internal conductive traces. The write driver, interconnect and write element create a write circuit. The electrical signal in the write circuit typically has two components, a differential signal and an undesirable common mode signal. The common mode component is typically created by noise. The common mode does not have a return path in the write circuit and thus behaves as an open circuit stub, with series and parallel resonance occurring at one-quarter and one-half wavelengths. The common mode noise can cross-talk onto the read traces. Additionally, the common mode can build charge on the write element. To alleviate the charge at the write element a trace can be connected to a write element center tap and ground. Such an approach can damp DC common mode but does not address AC common mode signals. Another approach is to ground both ends of the write element through resistors. The resistor values must be properly matched with the even and odd common mode suspension impedances to prevent impedance mismatch and resulting signal ringing, etc.